This invention relates in general to artificial hearts and more particularly to an artificial heart system that will respond to varying physiological demand and includes mechanisms accommodating the actual flow imbalance between pulmonary and systemic circulations.
Despite steady progress in developing a permanent artificial heart for long term implantation in a patient as a substitute for a failed natural heart, a number of issues must still be resolved. Among the issues that need to be addressed in an untethered artificial heart system are control strategies that respond to varying physiological demand, and mechanisms for accommodating the flow imbalance between the pulmonary and systemic circulations.
Left-right cardiac output differences have been well documented. Physiologically, the volume of blood flow pumped by the left side of the heart is higher than that pumped by the right side of the heart. This difference is largely attributable to a circulatory pathway known as the bronchial shunt. This flow originates in the left arterial system, passes through the bronchial tissue and then returns directly to the left atrium. This difference typically appears to be up to about ten percent of cardiac output with the left side flow always greater than the right side flow. Artificial heart systems must account for this inherent physiological circulatory imbalance. In addition, sources of flow imbalance can be man-made. For example, differences in regurgitation through artificial valves provided on the left and right sides can introduce a flow imbalance. Artificial heart systems must account for these types of circulating imbalances as well.
Pressure in the right atrium (RAP or right atrial pressure) is generally determined by the venous return into the heart and cardiac output. Average RAP typically ranges from 3 to 15 mmHg, depending on the physiology of the individual and that individual""s current activity level. Average LAP (left atrial pressure) typically ranges from 3 to 18 mmHg. In a biventricular cardiac prosthesis, controlling LAP within a physiologic range is important. If LAP is consistently high, not only can the atrium itself be damaged, but the high pressure can, in extreme cases, result in pulmonary edema or excessive fluid retention of the lungs. Also, when LAP is too low, atrial damage, air emboli and in-flow limitations can result. It is thus an important goal of any flow balance control in a biventricular cardiac prosthesis system to maintain LAP within a physiologic range. It can also be beneficial to balance pressure between the left and right sides of an artificial heart to maintain LAP within a physiologic range, both to maintain LAP and RAP in balance, and also to regulate LAP without directly measuring LAP. Direct measurement of LAP can be problematic, however, because it generally involves placement of a pressure transducer directly in contact with blood flowing through the left atrium, with the attendant potential problems such as thromboembolism.
In one known flow balance approach, controlled outflow valve regurgitation in the right chambers of one artificial heart is employed. (Lioi, A. P.; Kolff, W. D.; Olsen, D. B.; Crump, K.; Isaacson, M. S.; and Nielson, S. D.; xe2x80x9cPhysiological Control of Electric Total Artificial Heartsxe2x80x9d, in Devices and Contractors Branch Contractors Meeting 1985, Program and Abstracts, December 1985, 89.) In this approach the left and right sides of the heart are pumped alternately by a reversing hydraulic pump and a deliberate outflow leakage is designed into the right pump to accommodate the flow difference. However the regurgitant flow is only a function of the square root of the difference between the pulmonary diastolic pressure and the right atrial pressure. This results in a near constant compensating flow which may well be inadequate to accommodate differences. Also, changes in the orifice size over the long duration can cause this flow imbalance to drift from the preset value.
A second prior art approach employs a gas compliance chamber and passive filling to accommodate the flow difference in conjunction with a stroke-time division scheme. (Rosenberg, G.; Snyder, J.; Landis, D. L.; Geselowitz, D. B.; Donachy, J. H.; and Pierce, W. S., xe2x80x9cAn Electric Motor-Driven Total Artificial Heart: Seven Months Survival In The Calfxe2x80x9d, Trans Am Soc Artif Intern Organ, 15, 69, 1984.) The use of a gas compliance chamber leads to maintenance problems in maintaining the composition and pressure of the gas over time.
A further method and system for derating right side flow is described in Kung et al, U.S. Pat. No. 4,888,011 (incorporated herein by reference in its entirety). Kung provides a biventricular cardiac prosthesis having left and right side hydraulic chambers and left and right side blood pumping chambers which replace the natural ventricles in a patient with a failing heart. Also included is a reciprocating hydraulic pump that pumps hydraulic fluid back and forth between the right and left hydraulic chambers to drive right and left systole. Kung further provides a hydraulic compliance chamber having a flexible membrane coupled to the left atrial blood. During periods of higher left atrial pressure, hydraulic fluid is at least partially shunted to the hydraulic flow of the right pump, thus reducing the stroke volume and output of the right side. The Kung device relies on pressure in the hydraulic pumping chambers for pressure representative of respective atrial pressures. While this approximation of atrial pressure is sufficient to improve flow balance under a variety of conditions, the approximation can be less accurate under high blood flow conditions.
The invention provides a biventricular cardiac prosthesis having a flow control system for maintaining left atrial pressure within physiologic bounds. That is, the prosthesis flow control is responsive to left atrial pressure to maintain that pressure within physiologic bounds. In one aspect of the invention, a method for controlling a biventricular cardiac prosthesis measures a patient""s left atrial pressure, determines whether the left atrial pressure is outside of a desired tolerance about a desired left atrial pressure value, and derates right side blood flow to maintain left atrial pressure within the tolerance. An apparatus of this aspect of the invention includes left and right pumping sections, a right pumping section pumping volume derating element, and a control element controlling the derating of right side flow to maintain a patient""s left atrial pressure within physiologic bounds. The measurement of left atrial pressure may be taken directly, or indirectly as described hereinbelow.
In another aspect, the invention provides a method and apparatus for balancing flow by derating right side flow to hold a patient""s left atrial pressure close to the patient""s right atrial pressure, that is, the difference between left atrial pressure and right atrial pressure is close to zero. An apparatus of this aspect is a cardiac prosthesis that includes left and right hydraulic pumping chambers and a reciprocating hydraulic pump for alternately driving left and right systole. A control signal can be derived from the difference in pressure between the left and right hydraulic pumping chambers, and the control signal is applied to adjust the flow of the right hydraulic pumping chamber to maintain the control signal close to zero.
In another aspect, the invention provides a system and method for measuring the difference between right and left atrial pressures in a patient having a biventricular prosthesis. This system includes a prosthesis selected so as to have similar flow or pressure drop characteristics for blood inflow on its left and right sides and has one or more pressure transducers for measuring average diastolic pressures in its left and right pumping chambers. The difference between left and right atrial pressure in a patient having the prosthesis installed can then be determined from the difference in pressure between the left and right pumping chambers. In one embodiment, an energy converter having a hydraulic pump and a fluid switch drives left and right systole in the prosthesis. A single pressure transducer is provided in the hydraulic pump inlet to measure both left and right diastolic pressures. In this embodiment of the invention, the use of a single transducer to measure both pressures and then taking the difference between the two signals results in the desired measurement signal while eliminating any inaccuracies due to xe2x80x9cdriftxe2x80x9d that may occur in the transducer over time.
In a second embodiment of this aspect of the invention, the hydraulic pump is stopped for a short period of time to allow the hydraulic pressure in the left and right pumping chambers to equilibrate. Separate transducers measuring left and right hydraulic pressures are then polled and a difference is taken as an offset. This offset is then applied to difference measurements taken during operation of the prosthesis as a representation of transducer drift over time. In still another embodiment of this aspect of the invention, the speed of the hydraulic pump is varied according to a profile rather than stopped at the end of systole. Because this difference between left and right side pressure at this time is proportional to the square of the pump speed (which is known), with two or more measurements at different speeds, the pressure difference at a pump speed or zero (the transducer offset) can be extrapolated.
In still another aspect of the invention, a total artificial heart with controlled left and right flow is provided. The total artificial heart has left and right pumping sections, each having a blood pumping chamber connectable to a patient""s atrium for blood inflow, and a hydraulic pumping section. A reciprocating pump causes hydraulic fluid to flow back and forth between the right and left hydraulic pumping sections. A hydraulic balance chamber responsive to the patient""s left atrial pressure is in hydraulic communication with the right hydraulic pumping section by means of a hydraulic coupling with flow through the coupling between the right hydraulic pumping section and the hydraulic balance chamber affecting the stroke volume of the right hydraulic pumping section. The hydraulic coupling has a variable flow resistance that is adjustable to maintain a desired control of flow between the right and left pumping sections. In one embodiment, the resistance to flow in the coupling is adjusted by adjusting an occluder that effectively restricts a cross-sectional dimension of the coupling to increase resistance to flow, or alternatively increases a cross-sectional dimension to reduce resistance to flow.
In a further specific embodiment using active flow control, the total artificial heart is designed as described above so that the difference between left and right pumping section pressure is representative of the difference between left and right atrial pressure in a patient having the total artificial heart implanted. A signal representing the difference between left and right hydraulic pumping section pressures can then be used to automatically adjust the occluder to modify the right hydraulic section stroke volume to maintain the patient""s left atrial pressure close to the patient""s right atrial pressure, and thus within physiologic bounds.
In a further aspect of the invention, passive control is employed to maintain a patient""s left atrial pressure within physiologic bounds. In one embodiment of this aspect, a total artificial heart has left and right pumping sections, with a hydraulic balance chamber hydraulically coupled to a hydraulic pumping section of the right side. The hydraulic coupling includes a variable flow resistance that is adjustable to maintain balanced flow between the right and left pumping sections. The variable flow resistance can be provided by a variable orifice having a movable flow restrictor. The movable flow restrictor has a first side coupled to a pressure source representative of left atrial pressure and a second opposed side coupled to a pressure source representative of right atrial pressure so that the movable flow restrictor moves in response to differences between the two atrial pressures.
In a further embodiment, the hydraulic balance chamber is coupled to a patient""s left atrium to serve as a source representative of left atrial pressure, and the right hydraulic section serves as a source representative of right atrial pressure.